Photosynthetic microorganisms tend to form seasonal blooms in water bodies such as ponds, lakes, wastewater reservoirs and oceans. These blooms are defined by a massive increase in cell-number that can reach to up to 106-107 cells/ml and over than 50 μg/l chlorophyll-a. The phenomenon can be apparent to naked eye when water turns dark-green, red or brown. Although the bloom consists mainly of microorganisms capable to convert light energy via photosynthesis, it also coexists with a variety of many other microorganisms that support the entire community (Gardes et al., 2010). In some cases due to biotic and abiotic conditions, some of the species, namely blue-green algae, better known as cyanobacteria, use their gas vesicles to position themselves on water surface to form biofilms (also described as scums or mats). Cyanobacteria are a diverse group of oxygenic photosynthetic prokaryotes that possesses a versatile physiological and wide ecological tolerance, which contributes to their competitive success over a broad spectrum of environments. For more than 40 years the abundance of these organisms intensified globally in lakes, reservoirs, rivers and brackish water environments. Their blooms produce a musty odor and, more seriously, toxins. Cyanobacterial harmful algal blooms (CHABs) indicate an alert situation to water authorities, environmental and health agencies, as they reflect eutrophication problems as well as secretion of a huge array of metabolites, some of them most toxic to eukaryotes (Kaplan et al., 2012).
Several means for treating algal blooms are presently employed, e.g. a sequential treatment with aluminum sulfate (Al2(SO4)3) and copper sulfate (Hullebusch et al. 2002), peroxides (Drabkova et al., 2007), or herbicides (i.e. diuron, simazine, atrazine). However, these treatments are associated with grave environmental consequences (Falconer et al., 1983; Kolmakov, 2006; Swaen et al., 1992), and are also expensive (treatment with Al2(SO4)3 is estimated at US$750,000 to US$1,000,000 per sq km).
Accordingly, the available treatments are mostly used in artificial small ponds, pools and small shallow lakes where the ecological aspect of overdosing is not crucial. These treatments are not adequate for large water bodies, nor are they suitable for repeated use due to toxicity and ecological impact, their relatively high cost and the requirement for very large inputs for their uniform dispersal.
Another means for preventing CHABs is dumping barley or rice straw into aquatic ecosystems, though the effect was not consistent (Iredale et al., 2012). A recent discovery of their active compounds (flavonolignans salcolin A and B) (Xiao et al., 2014) demonstrated their lytic efficacy over Microcystis aeruginosa. However, these active compounds are not commercially available and are yet to obtain the required environmental and regulatory clearances.
Cyanophages, lytic viruses that specifically attack cyanobacteria, were hypothetically suggested, but were never practically identified or used (Deng and Hayess, 2008).
Matthijs et al., (2012) used a dispersal device and treated the whole volume of a shallow 0.12 sq km lake with ˜60 μM liquid H2O2. The procedure was expensive due to the high cost of the device, time consuming and risky due to the use of liquid hydrogen peroxide. Also, the chemical was recorded in the water two days post treatment. Moreover, frequent exposure to H2O2 in large scale areas and at high frequencies may induce cyanobacterial resistance to the compound in the long run.
Chironomids (Diptera; Chironomidae; Chironomus) are the most abundant species of insects in freshwater worldwide. They undergo a complete metamorphosis in four life stages; three are aquatic (eggs, larvae, pupae) and the last is the terrestrial adult stage. Females lay egg-masses (of 400-1,000 eggs) embedded in a thick gelatinous matrix on the edge of water bodies.
The Chironomids (also known as non-biting midges) cause severe ecological and economical nuisance. Massive swarms of adult Chironomids emerging from aquatic habitats near, or in urban areas affect tourism and real estate value and are associated with human allergic reactions. As larvae they clog water pipes and may reach the water supply systems of home-users (“red worms”). Furthermore, Chironomids' gelatinous egg-mass is reported to serve as a natural reservoir of Vibrio cholera (Broza and Halpern (2001)). Currently, the efforts to prevent cholera rely on a combination of sanitary hygienic measures. Pesticides, which are used against the chironomids' larvae, have limited success since in a chronic use, the midges were found to adjust and become resistant. Moreover, pesticides have a broad specificity and may harm the environment, including humans.